These Little Lights of Mine

14,000 quasars shine a light on the distant universe

A slice through the 3-D map. We are at the bottom of the wedge. Distances are
shown in billions of light-years.

The black dots are nearby galaxies. The colored
region shows the map of intergalactic hydrogen gas. Red means more gas; blue means
less. The red cross-hatched region was not seen in this study.

Figure credit: A. Slosar and the SDSS-III collaboration

Scientists from the Sloan Digital Sky Survey (SDSS-III) have
created the largest ever three-dimensional map of the distant universe by
using the light of the brightest objects in the cosmos to illuminate
ghostly clouds of intergalactic hydrogen. The map provides an
unprecedented view of what the universe looked like 11 billion years ago.

The new findings were presented today at a meeting of the American
Physical Society by Anže Slosar, a physicist at the U.S. Department of
Energy's Brookhaven National Laboratory, and described in an article
posted online on the arXiv astrophysics preprint server.

The new technique used by Slosar and his colleagues turns the standard
approach of astronomy on its head. "Usually we make our maps of the
universe by looking at galaxies, which emit light," Slosar explained. "But
here, we are looking at intergalactic hydrogen gas, which blocks light.
It's like looking at the moon through clouds - you can see the shapes of
the clouds by the moonlight that they block."

Instead of the moon, the SDSS team observed quasars, brilliantly luminous
beacons powered by giant black holes. Quasars are bright enough to be seen
billions of light years from Earth, but at these distances they look like
tiny, faint points of light. As light from a quasar travels on its long
journey to Earth, it passes through clouds of intergalactic hydrogen gas
that absorb light at a specific wavelengths, which depend on the distances
to the clouds. This patchy absorption imprints an irregular pattern on the
quasar light known as the "Lyman-alpha forest."

An observation of a single quasar gives a map of the hydrogen in the
direction of the quasar, Slosar explained. The key to making a full,
three-dimensional map is numbers. "When we use moonlight to look at clouds
in the atmosphere, we only have one moon. But if we had 14,000 moons all
over the sky, we could look at the light blocked by clouds in front of all
of them, much like what we can see during the day. You don't just get many
small pictures -- you get the big picture."

A zoomed-in view of the map. Red means more gas; blue means less gas. The black scalebar
in the bottom right measures one billion light years.

Figure credit: A. Slosar and the SDSS-III collaboration

The big picture shown in Slosar's map contains important clues to the
history of the universe. The map shows a time 11 billion years ago, when
the first galaxies were just starting to come together under the force of
gravity to form the first large clusters. As the galaxies moved, the
intergalactic hydrogen moved with them. Andreu Font-Ribera, a graduate
student at the Institute of Space Sciences in Barcelona, created computer
models of how the gas likely moved as those clusters formed. The results
of his computer models matched well with the map. "That tells us that we
really do understand what we're measuring," Font-Ribera said. "With that
information, we can compare the universe then to the universe now, and
learn how things have changed."

The quasar observations come from the Baryon Oscillation Spectroscopic
Survey (BOSS), the largest of the four surveys that make up SDSS-III. Eric
Aubourg, from the University of Paris, led a team of French astronomers
who visually inspected every one of the 14,000 quasars individually. "The
final analysis is done by computers," Aubourg said, "but when it comes to
spotting problems and finding surprises, there are still things a human
can do that a computer can't."

"BOSS is the first time anyone has used the Lyman-alpha forest to measure
the three-dimensional structure of the universe," said David Schlegel, a
physicist at Lawrence Berkeley National Laboratory in California and the
principal investigator of BOSS. "With any new technique, people are
nervous about whether you can really pull it off, but now we've shown that
we can." In addition to BOSS, Schlegel noted, the new mapping technique
can be applied to future, still more ambitious surveys, like its proposed
successor BigBOSS.

When BOSS observations are completed in 2014, astronomers can make a map
ten times larger than the one being released today, according to Patrick
McDonald of Lawrence Berkeley National Laboratory and Brookhaven National
Laboratory, who pioneered techniques for measuring the universe with the
Lyman-alpha forest and helped design the BOSS quasar survey. BOSS's
ultimate goal is to use subtle features in maps like Slosar's to study how
the expansion of the universe has changed during its history. "By the time
BOSS ends, we will be able to measure how fast the universe was expanding
11 billion years ago with an accuracy of a couple of percent," McDonald said.
"Considering that no one has ever measured the cosmic expansion rate so
far back in time, that's a pretty astonishing prospect."

Quasar expert Patrick Petitjean of the Institut d'Astrophysique de Paris,
a key member of Aubourg's quasar-inspecting team, is looking forward to
the continuing flood of BOSS data. "Fourteen thousand quasars down, one
hundred and forty thousand to go," he said. "If BOSS finds them, we'll be
happy to look at them all, one by one. With that much data, we're bound to
find things that we never expected."

About SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation,
the Participating Institutions, the National Science Foundation, and the
U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.
SDSS-III is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS-III Collaboration including the
University of Arizona, the Brazilian Participation Group, Brookhaven
National Laboratory, University of Cambridge, University of Florida, the
French Participation Group, the German Participation Group, the Instituto
de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, New Mexico State
University, New York University, Ohio State University, Pennsylvania State
University, University of Portsmouth, Princeton University, the Spanish
Participation Group, University of Tokyo, University of Utah, Vanderbilt
University, University of Virginia, University of Washington, and Yale
University.

SDSS-III is managed by the Astrophysical Research Consortium for the
Participating Institutions of the SDSS-III Collaboration including the
University of Arizona, the Brazilian Participation Group, Brookhaven
National Laboratory, University of Cambridge, University of Florida, the
French Participation Group, the German Participation Group, the Instituto
de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National
Laboratory, Max Planck Institute for Astrophysics, New Mexico State
University, New York University, Ohio State University, Pennsylvania State
University, University of Portsmouth, Princeton University, the Spanish
Participation Group, University of Tokyo, University of Utah, Vanderbilt
University, University of Virginia, University of Washington, and Yale
University.